NBSIR 74-360
A DYE MODE-LOCKED Nd'^ GLASS LASER FOR
GENERATING ELECTRICAL REFERENCE WAVEFORMS
Tatsutoku HondaNorris S. Nahman
Electromagnetics Division
Institute for Basic StandardsNational Bureau of StandardsBoulder, Colorado 80302
September 1972
NBSIR 74-360
A DYE MODE-LOCKED Nd GLASS LASER FOR
GENERATING ELECTRICAL REFERENCE WAVEFORMS
Tatsutoko HondaNorris S. Nahman
Electromagnetics Division
Institute for Basic StandardsNational Bureau of StandardsBoulder, Colorado 80302
September 1972
iT* W ^ ^4
U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary
NATIONAL BUREAU OF STANDARDS. Richard W Roberts, Director
FOREWORD
This report summarizes the research performed by Mr. T.
Honda during his stay at the National Bureau o£ Standards,
Boulder, Colorado, as a visiting guest scientist from
September 1971 to September 1972. Mr. Honda's home labora-
tory is the Electrotechnical Laboratory, Tokyo, Japan. This
report is typical of the high quality research performed in
the international scientific exchange programs that NBS parti-
cipates in and strongly supports. The major publication from
this research was "The Optical Impulse Response of Biplanar
Vacuum Photodiodes , " coauthored by Mr. Honda and Dr. R. Smith
of NBS. The paper was published July, 19 73 in Applied Optics.
The second author Dr. Norris S. Nahman was the section
chief of the Pulse and Time Domain Section in which Mr. Honda
performed his research. Dr. Nahman provided scientific
counseling and direction of this research effort. He is
no longer with NBS. He is presently the chairman of the
Electrical Engineering Department of the University of Toledo,
Toledo, Ohio.
James R. Andrews
January 19 74
CONTENTS
Page
1. INTRODUCTION 1
2. CUTLER MODEL FOR SATURABLE DYE ABSORBER MODE -LOCKEDLASER 5
2.1 Linear Active Medium 5
2.2 Saturable Absorber 7
3. DYE MODE -LOCKED Nd"^^ GLASS LASER 10
3.1 Geometry 10
3.2 Pump Energy Charging and Discharging Circuit 11
4..
OPERATION OF THE Nd"*"^ GLASS LASER 12
4.1 Alignment of the Laser System 12
4.2 Concentration o£ Absorber Dye 12
4.3 Excitation Energy 13
4.4 Optical Damage 14
4.5 Time Interval of Operation 14
5. STABILITY OF DYE MODE -LOCKED LASER 15
5.1 Laser Rod 15
5.2 Excitation 16
5.3 Degradation of the Bleachable Dye 16
6. SINGLE PULSE EXTRACTION AND MEASUREMENT 20
6.1 Pulse Extraction 20
6.2 Two Photon Fluorescence Pulse Measurements 20
iii
CONTENTS (Continued)
Page
7. GENERATION OF ELECTRICAL PULSE WAVEFORM OF KNOWNSHAPE 23
7.1 Introduction 23
7.2 Pyroelectric Detector - 23
7.3 Biplanar Vacuum Photodiode - 25
7.4 Optical Rectification 26
REFERENCES- 29
LIST OF FIGURES
Figure 1. A Model for Electronic and Optical Regenera-tive Pulse Generators — 34
Figure 2a. Optical Nonlinear Element for Mode LockedLaser 35
Figure 2b. Pulse Sharpening Due to Repeated Traversals ofthe Feedback Loop 35
Figure 3. Laser Model for Cutler Analysis 36
+ 3Figure 4. Mode Locked Nd Glass Laser 37
Figure 5. Conventional Laser and Mode Locked LaserOutputs 38
Figure 6. Laser Flash Lamp Electronics 39
Figure 7. Laser Burn Patterns 40
Figure 8. Shot to Shot Instability of Mode Locking 41
Figure 9. Excitation Energy Dependence of Mode Locking-- 42
Figure 10. Fatigue of Dye 43
Figure 11. Measurement of Intensity Dependence of Trans-mission of 9860 Dye 44
iv
LIST OF FIGURES CContinued)
Page
Figure 12. Optical Attenuation Versus Intensity of 9860Dye (T=6%) 45
Figure 13. Transmission Property of 9860 Dye 46
Figure 14. Change of Transmission Property of 9860 DyeResulting from Mode Locked Laser Operation 47
Figure 15. Small Signal Transmission of 9860 Dye VersusExposure Time Under Blue Light 48
Figure 16. Single Pulse Extraction Technique 49
Figure 17. Single Pulse Generation Waveform 50
Figure 18. Two Photon Fluoresence Experimental Setup 51
Figure 19. Two Photon Fluoresence Results 52
Figure 20. Two Photon Fluoresence Results 53
Figure 21. Generation of Picosecond Pulsed Waveform ofKnown Shape 54
Figure 22. Equivalent Circuit for a Pyroelectric Detector 55
Figure 23. Pyroelectric Detector Waveforms 56
Figure 24. Pyroelectric Detector 57
Figure 25. Biplanar Photodiode 58
Figure 26. Equivalent Circuit for Biplanar Photodiode 58
Figure 27. Impulse Response of Biplanar Photodiode 58
V
A DYE MODE-LOCKED Nd"^^ GLASS LASER FOR GENERATING
ELECTRICAL REFERENCE WAVEFORMS
T. Honda* and N.S. Nahman**
The theory, design, construction, operation andstability o£ dye mode-locked Nd"*"^ glass laser are dis-cussed. Optical properties of a saturable dye, singlepulse generation and two-photon fluorescence are ex-perimentally studied. For the application of pico-second optical pulses to the field of the basebandpulse measurements, three types of modelable detec-tors are described which have possible applicationsfor the generation of electrical reference waveforms.
+ 3Key words: Glass laser; laser; mode-lock; Nd;
picosecond; reference waveform.
1. INTRODUCTION
The purpose of this work was to develop a pulsed laser
for generating optical picosecond pulses. The laser is to
be used as an optical pulse source in a study on the genera-
tion of electrical pulses by optical pulses. A mode-locked
+ 3Nd glass laser was chosen for this work because a glass
laser is able to generate the shortest pulses and large
peak power [ 1]
.
*Mr. Honda was an NBS guest scientist during 1971-72. He iswith the Laser Research Section, Radio § Opto-ElectronicsDivision, Electrotechnical Laboratory, Tanashi Branch,Tan as hi, Tokyo, Japan.
**Dr. Nahman is presently with the Electrical EngineeringDept., University of Toledo, Toledo, Ohio.
The generation of large output pulses from laser devices
was first achieved by McClung and Hellworth [2] in 1962, This
technique is now called laser-Q switching; the shortest pulse
available from a Q-switched laser is about 10 nsec. By the
mode- locking technique it is now possible to produce 0.1 psec
pulses [3] at powers up to 100 Gigawatts.
The mode-locked laser pulse generator can be charac-
terized as a regenerative pulse generator. It is well known
that a feedback loop consisting of an amplifier, a filter,
a delay line and a nonlinear element (which provides less
attenuation for a high-level signal than for a low- level
signal) behaves as a regenerative pulse generator, figure la.
In 1955, Cutler [4] discussed the regenerative pulse genera-
tor as a microwave pulse generator and was able to generate
microwave pulses having 2 ns pulse duration at 4 GHz.
Referring to figure la, when the loop-gain exceeds
unity, a pulse recirculates indefinitely around the loop
and each traversal gives rise to an output pulse at the
output terminal. The nonlinear element called an "expandor"
by Cutler has the three following characteristics:
1) Exhibits low loss for the peak region of the re-
circulating pulse while a high loss for lower amplitude
region
.
2) Discriminates against noise and reflections.
2
3) Acts to shorten the pulse duration until the pulse
duration is limited by the inherent bandwidth of the
filter.
The output of the regenerative oscillator has a pulse repe-
tition rate equal to the reciprocal of the loop delay, a
pulse duration equal to the reciprocal of the system band-
width, and a carrier frequency determined by the filter
frequency.
A mode-locked laser pulse generator is an optical re-
generative pulse generator, figure 1. The laser medium
serves as the amplifier, the combination of the Fabry-Perot
resonator and the line width of the laser transition serves
as the filter, and the time required for an optical pulse
to transverse twice the distance between the mirrors serves
as the loop time delay.
The optical analog of the "expandor" is a saturable
absorber, such as the reversible bleachable dye solution
commonly used in laser Q-switches . The three fundamental
requirements for a saturable absorber are as follows:
1) An absorption line at the laser wavelength.
2) A line width equal to or greater than the laser
line width.
3) A dye recovery time which is shorter than the loop
time delay of the laser.
3
Such a nonlinear element, figure 2a, sharpens pulses upon
repeated traversals of the feedback loop. The pulses become
successively peaked until a steady state is reached, figure
2b.
The steady state represents a balance between the nar-
rowing caused by the nonlinear element and the spreading out
of the pulse due to the finite line width (frequency band-
width) of the laser material. The study of short pulse forma-
tion in the laser has been approached from two theoretical
directions [5-8]. One method is based upon detailed studies
of the laser cavity modes [5,7,8]. Such an approach leads
to difficult analytical or numerical computation, because
of the complicated sets of coupled mode equations. However,
such studies have had notable success in explaining short
pulses in terms of the locking of the phases of the modes.
The other approach is simpler and is based upon the
method used by Cutler in studying microwave pulse generators
[4].
The Cutler method of analysis is in terms of functions
of time, and it has advantages both in terms of the physical
picture and of the mathematical operations involved. Also,
the Cutler method of analysis applied by Creighton, et al
.
[6] is described here because their analysis has rather
general forms for the functions representing operations on
signal in the laser, whereas most other analyses assume
these functions to be Gaussian.
2. CUTLER MODEL FOR SATURABLE DYE ABSORBER MODE-LOCKED LASER
2 . 1 Linear Active Medium
The laser is assumed to be operating on a particular line
o£ central frequency and approximate width fiw, figure 3.
A ring laser is imagined to simplify considerations. The
light transverses a complete cycle and returns to the input
side of the active medium.
Assuming that the active material is not saturated, thus
the electric field from it is a linear function of the inci-
dent field; also its response is assumed not to vary with time.
A linear operator represents both the linear operation of the
active medium on the signal, and the lumped losses in the
entire optical circuit. The linear operator is taken to have
a transfer function F((ja) . Referring to figure 3, if the in-
put signals at A is I(t) and the output signal at B is 0(t)
,
then
00
0(t) = / rCco) • e"^'^''^ do) (1)oo
where T(a3) is the Fourier transform of I(t) and 7(a)) is the
transfer function of the linear operator. From (1)
oo
0(t) = / F(t-t') 1(f) dt' (2)
5
The response function
- 00
which has a response time p on the order o£ magnitude o£ the
reciprocal o£ the bandwidth. 0 in a narrow range
about the optical frequency o) . Explicitly in terms of u) ,
F((jo) has the form
F(oo) = M'CciO-oo ) + fT*C-aj+co ) = MC^) + FT*(-J^) (3)c c
M* results from the requirement that linear transfer functions
must satisfy the condition F((jo) = F*(-oo),
If the initial carrier pulse is
G(t) = G^^(t)cos oa^t (4)
and is such that G^^(t) varies slowly with respect to the
carrier frequency, then the output pulse envelope can be
approximated by_
/ ' '
'
00
G ^(t) = / F(t-t')G. (f) dt' (5)— 00
which varies slowly compared to optical carrier frequency.
We have the approximate result that the filter acts on solely
the envelope function as if its transfer function were M(f2) ,
FCt) = ^ / M(J^)e^"^ dt (6)-00
6
The width of the response function F(t) is designated by the
symbol p where is the variance of F(t) (see (11)). The
time p is long compared to the carrier period, but short
compared to the cycle time of the laser.
Unfortunately, there is no simple way of writing the
output from a bleachable dye in terms of its input. The
expandor in Cutler's paper was assumed to act on the ampli-
tude of the pulse envelope by a power of S, where S > 1. Al-
though a bleachable dye does not behave in this simple way,
it does generally narrow pulses which pass through it as does
Cutler's expandor. A pulse going through a laser gives rise
to the next pulse; the signal at the output C is
where (t) is the j-th pulse at the input A. The input for
the (j+1) pulse is
2.2 Saturable Absorber
(7)
A
j+1(t) = / F(t-T^^-t')G^(t') (8)
2 . 3 The Steady St ate
The steady state condition is given by
dt (9)
7
where
^' = T^C = f - (10)
Here T is the total time for a pulse to travel around the
loop; T^^ is the time for travel from A to C, while T^^ is
that for C to A.
For a criterion for the pulse duration, we use the
variance defined by
00
/ Ct-<t>^)f(t) dt
= :^— (11)
/ £Ct) dt-00
where
00
, . / tf(t) dt
<t> =
/ £Ct) dt-00
This is a convenient criterion because variances add under
convolution, i.e., for
00
f(t) = / g(t-f)h(t') dt', (12)- 00
we have /
From (9) we have
oo
G(t-A')^/^ = / F(t-t')G(t') dt'
8
Furthermore, if G(t) is a Gaussian function, then the variance
1/Sfor G(t) ' is the variance of G(t) multiplied by S. Denoting
the variances of G(t) and F(t) by and p^, respectively, then
= — (14)S - 1
where P - ^ (approximately the lifetime of the gain medium
line). When S = 2 , we have a square law saturable absorber
and (14) reduces to
" = p = L' (1=)
which says that the pulse width p is approximately equal to
the reciprocal of the gain line bandwidth. Cutler's analysis
[4] yields a similar result.
In the case of a Gaussian filter function and pulse shape,
and a square law saturable absorber, Cutler finds for the pulse
duration
,
S/ttT =
Ao)
1 Ml + )
(16)
18
where y is the coefficient for the nonlinear effect originating
from the dispersion, and Ago is the bandwidth for a Gaussian
filter defined by the response at the minus -one neper points.
If y is small enough,
which is the same form as (15) .
9
3. DYE MODE-LOCKED Nd"^^ GLASS LASER [9], [11]
3 . 1 Geometry
A schematic diagram of the dye mode- locked laser is shown
in figure 4. The laser produces an output at 1.06 ym which
is invisible and in the infrared range. The laser glass rod
is 1.6 cm in diameter and 19.3 cm in length; the rod is
+ 3brewster cut at both ends; and made of Nd doped laser glass.
The laser rod is housed in the center of a double elliptical
pumping cavity having a gold-polished surface , containing two
straight Xenon lamps. 14.5 cm of the laser rod is in the
pumping cavity. The mirrors are wedged and anti-reflection
coated on the back surfaces at 1.06 ym. In some experiments,
we used an output mirror having a lower reflectivity of
60% in order to change the threshold of lasing. Also the
other mirror had a radius of curvature of 10 m to form a
stable resonator. The saturable dye (9860) is housed in a
thin cell constructed with a 1.65 mm PTFE spacer; the dye
was dissolved in 1 , 2- dichloroethane and the transmission
adjusted to be 75 to 80 percent. The dye was put in close
proximity to the 100% mirror [12]. The laser emission shown
in figure 5 is observed with a photodetector (Biplanar Vacuum
Photodiode) and a traveling-wave oscilloscope (over all rise-
time 0.55 nS) . It has the form of a train of approximately
50 short pulses spaced by about 10 ns , the cycle time T of
the laser.
10
3 . 2 Pump Energy Charging and Discharging Circuit
The block diagram of the laser flashlamp pumping system
is shown in figure 6. The capacitor bank consists of ten
capacitors of 25 yf parallel connected so as to store
1000 joules when a charging voltage of 2.8 kV is applied.
The Xe lamp is triggered in a series mode [13] by using a
pulse transformer having a saturable inductance, which also
serves as a current limiter.
11
4. OPERATION OF THE Nd"*"^ GLASS LASER
4.1 Alignment of the Laser System
Alignment of the system initially is done by using the
visible beam of a cw He-Ne laser (0.6 ym) as a "guiding-
string." It is rather hard to make a good alignment using
a simple serial alignment on the He-Ne beam. A better proce-
dure is to employ a Fabry- Perot interference pattern from the
incident side of the resonator where the screen on which the
pattern images is kept as far as possible from the resonator.
The reflected beams from the mirrors should be completely
superimposed by adjusting mirrors while watching the inter-
ference pattern. However, it should not be expected that
the alignment will be perfect before lasing because the lasing
wavelength (1.06 ym) will be different from that of the He-
Ne beam (0.6 ym) . The final adjustments must be made under
actual lasing conditions. The actual far-field pattern
will show whether or not the alignment has been done
correctly and is obtained from burn patterns produced by
the laser beam, figure 7.
4.2 Concentration of Absorber Dye
The dye 9860 was chosen as a saturable absorber for mode-
locking because it has some advantages involving high satura-
tion power density, fast relaxation time and high damage
threshold, table 1.
12
For determination of the dye concentration, it is neces-
sary to consider the excitation energy capacity and optical
damage threshold of the rod and mirrors . Too high a concen-
tration of dye makes the lasing threshold of a mode-locked
laser too large; this in turn could require optical energy
stored inside the laser to be equal to or larger than the
optical damage threshold.
In the case of too low concentration of dye, the laser
would be poorly mode-locked.
4 . 3 Excitation Energy
Usually, excitation level is determined by a "cut and
try" method. The threshold of a mode-locked laser depends
upon the gain and losses of the whole laser system involving
the reflectivity of mirrors and the absorption of the dye.
It is necessary to avoid pumping near the threshold,
because too low pumping causes instability of lasing and
mode- locking, and is strongly influenced by the fluctuation
of the excitation power from Xenon lamps which can be 10 to
20 percent. Too high a pumping level can cause damage to
the laser components. Consequently, care must be taken in
increasing the pumping level to achieve stable operation.
13
4.4 Optical Damage
Optical damage on the surfaces of optical devices such
as mirrors and in bulk materials such as rods can be pro-
duced by high power mode-locked laser pulses. The surface
damage results from optical power absorbed at dirty spots
on surfaces of optical devices. Surfaces should always be
kept clean.
In bulk materials an excessively high power laser beam
can lead to self - focusing of the optical beam. To avoid the
production of "hot-spots" in the beam uniform pumping and
homogeneous dye solution are required.
4.5 Time Interval of Operation
The shot to shot time interval allowed depends on the
cooling time constant of an overall system and pumping energy
level. Generally speaking, regular [periodic) operation re-
sults in a good reproducibility of lasing; however, the
repetition rate must be slow enough so that the system cools
down to the same initial state. Typical repetition rates
would be of the order one shot/10 minutes for highest power
operation.
14
5. STABILITY OF DYE MODE -LOCKED LASER
Instability o£ a dye mode - locked laser is caused by
various conditions o£ the components in the laser system.
The main causes which impede stability of mode-locked laser
and the appropriate counter-measures are shown in table 2.
The detail about some of these causes will be discussed
below. A typical example of shot to shot instability of
dye mode-locked laser is shown in figure 8.
5 . 1 Laser Rod
The pumping energy heats the laser rod and produces a
temperature gradient throughout the rod. This gives rise
to a change of refractive index and mechanical distortion in
the rod if the distribution of pumping energy and absorption
is not uniform.
It is well known that glass is a poor thermal conductor.
It will take a considerable amount of time until the heated
rod will cool. For a simple estimation of the thermal dif-
fusivity, we have to take into account a thermal time constant
of about 30 to 50 sec for the glass material. If an energy
of 100 joules (equivalent to electrical input energy of 1000
joules applied to Xenon lamp of 101 efficiency) is absorbed
by the glass rod, the temperature would correspondingly rise
by 1.6 to 2.6°C.
15
5.2 Excitation
An example of excitation energy dependence of mode- locking
is shown in figure 9. From these pictures, the mode- locking
threshold can be observed.
By observing the intensity of 0.53 ym emitted from a
Xenon lamp, a shot to shot intensity fluctuation of 10 to 201
was found. The fluctuation is influenced by the change of
charging voltage, triggering, and physical situation of the
lamp. The electrically induced fluctuation troubles may
be minimized by utilizing an electronic automatic and internal
(series) triggering method for driving the Xenon lamps.
The physical situation of the lamp is complicated. The
emission around 0.5 8 y is most effective for lasing; however,
the spectrum emitted from a Xenon lamp depends upon temperature,
pressure, input energy and use -de gradation of the lamp. Over-
all, better shot to shot energy stability can be achieved by
regular operation at not too low a discharging voltage.
5 . 3 Degradation of the Bleachable Dye
It has been noted that the dye solution dominates the
stability of a saturable absorber mode-locked laser. Unfor-
tunately, only experience can provide the information for
establishing the generation of good mode-locked pulses.
16
The dye manufacturer suggests that the life of dye is
about 500 shots at optical power level of 1 to 5 MW. This
power level seems to be quite low for the laser system
discussed here. Experimentally poor mode-locking has been
observed after 30 to 60 shots, figure 10. The degradation of
the dye solution results in the change of its transmission
property, which destroys mode - locking . If the dye solution
loses its essential optical nonlinearity , no mode-locking can
be achieved.
The transmission property of a dye can be measured by
transmission experiments. The intensity dependence of the
optical absorption for a dye solution was measured by using
a train of short pulses from a mode-locked laser. A block
diagram of the measurement system is shown in figure 11.
To obtain an appropriate optical power density for
bleaching the dye, a lens having a focal length of 50 cm
was used. A dye cell 1.65 mm thick was placed away from the
lens by about 30 cm in order to give approximately the same
optical power density as that stored in the mode-locked laser.
The laser beam is split into two beams; one is used as a
reference and the other as the probe. The test cell has the
same size as one being used for mode - locking . Both beams are
detected by a vacuum photodiode (0.5 ns risetime) and the out-
put pulses are displayed by a traveling wave oscilloscope
CO. 35 ns risetime).
17
The intensity dependence of the optical absorption of the
9860 dye solution is demonstrated in figure 12; less absorp-
tion at high power can be seen. The results of the transmis-
sion measurements on the 9 860 dye solution are shown in figure
13. The solid-line curves are theoretical ones computed by
using the two-level model developed by Kercher [14].
For a thin cell mode (propagation through the cell < opti
cal pulse duration), Hercher gives for the transmission, T,
T = 1 - —
where a is the small signal absorption, and I^, is the optical
intensity at which absorption is Sq/Z.
The small signal transmission was measured separately
by using a combination of a white spectrum lamp, a spike
(transmission) filter for 1.06 ym and a photomultiplier . The
small signal transmission was adjusted to 37.51, 62.5%, and
78%. The curves of the thin cell model account well for the
experimental points for the weak absorbing dye solution. The
two-level model has been supported experimentally [15]. As
noted by the ref. [15], we have also observed a decrease of
the transmission for optical pulse intensities larger than
a certain value.
18
In figure 14, the change in the transmission property for
a 9860 dye solution resulting from mode-locked laser operation
is shown. Apparently the nonlinearity of the dye was weakened.
The dye solution was degraded in the mode-locked laser after
35 shots; note the increase in the small signal transmission.
The degraded dye could no longer provide the necessary mode-
locking nonlinearity.
In this case, the number of shots of mode-locking opera-
tion is not so important, but rather the change of the trans-
mission property of the degraded dye solution when it happened.
It should be also noted that the excitation power for mode-
locked lasing with fatigued dye is lower than with new dye.
Also the transmission property of dye solution is strongly
influenced by absorption of blue light and ultra-violet radia-
tion. The change of the small signal transmission of the dye
solution versus exposure time under a fluorescent lamp is
shown in figure 15. The completely bleached out state of
the dye is shown. It may be concluded that the degradation
of dye could be considerably influenced by blue and ultra-
violet light from Xe lamps.
19
6. SINGLE PULSE EXTRACTION AND MEASUREMENT
6.1 Pulse Extraction
A single pulse can be extracted from a train of pico-
second pulses by using an optical gate, figure 16. The gate
consists of an electroopt ical polarization switch placed be-
tween two polarizers. When the gating pulse is applied to
the polarization switch, the polarization of the optical
signal is changed; consequently, if the signal is then passed
through a polarization discriminator, the signal will be
spatially separated into its two distinct polarization
components. The gating pulse of 4 to 5 kV, 8 to 10 ns is
generated by a spark gap pulse generator which is triggered
by the laser beam. A typical picture of single pulse genera-
tion is shown in figure 17.
A single picosecond pulse is considered to be very useful
for electrical time domain studies. Furthermore, this tech-
nique makes it possible to obtain the single pulse having the
shortest duration in the train of optical pulses.
6 . 2 Two Photon Fluorescence Pulse Measurements
Optical coincidence techniques involving intensity auto-
correlation functions have been very useful in the study of
ultrashort optical pulses of a few picoseconds in duration.
20
A commonly used technique is that of two photon fluorescence
(TPF) [16] ; the block diagram for a TPF measurement of pulse
width is shown in figure 18. The laser beam is divided into
two beams of equal intensity; these interfere in a dye which
exhibits two-photon induced fluorescence. The pulse duration
is proportional to the width of the auto- correlation function.
The time - integrated fluorescence intensity F(t) excited
by the two identical wave packets V(t) propagating in the
opposite direction is given in terms of the correlation
function (x)
:
F(t) = 2F^[1+2G2(t)/G^(0)]
in which
G^Ct) = <VCt)VCt+T)> <V*(t)V*(t+T)>
where x = 2Z/V (V = group velicty of the wave packets and Z is the
position) and F^ the single pass fluorescence. The notation
<V(t)V(t+x)> denotes the integration
,. , T/2
T ! V(t)V(t-x) dt-T/2
The contrast ratio K of the fluorescence pattern is de-
fined as the ratio of the peak fluorescence at x = 0 to the
background fluorescence at large values of x;
K = F(0) /F(x->°=)
It is well known that K assumes the value of 3 in the absence
of background intensity and reduces to 1.5 for Gaussian noise
which is caused by the thermal radiation generated by a laser
beam.
21
Rhodamine 6G was utlized as the dye medium. In ethanol
solution, this dye has its primary absorption peak at a wave
length that is quite close to the 0.5 3 um, the second har-
+ 3monic of the Nd glass laser. In figures 19 and 20, some
typical pictures of TPF by mode- locked pulses are shown.
For TPF experiments, it is always necessary to adjust
carefully the optical beams to be completely superimposed in
optical power density. Too strong optical power increases
noise and reduces the contrast ratio.
The two-photon fluorescence method has considerable
ambiguity for determinating pulse duration because the pulse
waveform is unknown. Many workers have been trying to find
the waveform of mode- locked pulses and measure these pulse
duration. It is said that the front part of a mode-locked
pulse is expressed with Gaussian and the tail exponential.
In spite of the ambiguity of TPF, the method is very
useful for the estimation of picosecond pulse duration be-
cause of its simplicity and one shot measurement capability.
22
7. GENERATION OF ELECTRICAL PULSE WAVEFORM OF KNOWN SHAPE
7.1 Introduction
In the NBS Section 2 72.20, research has been done on
reference waveform generators and standards [17] to be used
in pulse measurements and other applications. The method
employs the band- limiting properties of a lossy uniform trans-
mission line to produce a known waveform and generator
impedance. An example of generator design employing planar
skin effect metal loss and debye dielectric loss with a unit
ramp generator is shown in figure 21a.
The ultimate application of the laser system described
in this report is to replace the convolution problem of the
combination of a unit ramp generator and a modeled line by the
combination of a picosecond optical pulse generator and a
modeled detector, figure 21b.
Three kinds of optical detectors have been considered for
the modeled detector having a predictable impulse response:
1) fast thermal photodetector ; pyroelectric effect, 2) biplanar
vacuum photodiode , and 3) nonlinear optical effect: optical
rectification.
7 . 2 Pyroelectric Detector
The pyroelectric effect [18, 21] concerns the change in
the polarization of a polar crystal when it undergoes a
variation in its temperature. Optical radiation is absorbed
by means o£ a surface block of an optically absorbing material
attached to a piece of spontaneously polarized pyroelectric
material. If the pyroelectric material serves as the di-
electric in a capacitor, then the resultant change of polari-
zation due to heating of the pyroelectric gives rise to a
pyroelectric voltage across the capacitor electrodes.
The fast thermal photodetector is expected to generate an
electrical pulse waveform having a risetime of 100 picoseconds.
The pyroelectric current is given by,
pcFdP
where P = ^tjt- , P^ : spontaneous polarization, T: temperature,
C: specific heat, b: thickness of pyroelectric material,
A: area of detector, AW: change of radiation power, p: density
of pyroelectric material. An equivalent circuit is shown in
figure 22. The output voltage is given by
V = kl0 0
1_ U~
^e
t t
1 , -t; t;^e - e
where Trr is thermal time constant, T circuit time constant1 e
(Rj^C) , and peak intensity.
In figure 23, the output signals to be expected are shown
for the pyroelectric detection process in cases of both impulse
and step incident optical beams. Generally, this detector has a
24
disadvantage of small signal output. An example of pyroelec-
tric photodete ctor design is shown in figure 24. Ni filmo
electrodes of 200 A thick were evaporated on both surfaces
of a PVF2 pyroelectric film of - 100 ym thick.
7 . 3 Biplanar Vacuum Photodiode
The impulse response of a biplanar vacuum photodiodes
[22, 24] was experimentally investigated [27] by the use of
a single ultrashort laser pulse from a train of mode-locked
pulses. The impulse response of a biplanar vacuum photodiode
for the observation of ultrashort optical pulses is determined
by the transit time of the photoelectrons from the photocathode
to the anode, figure 25.
Transit time of a photoelectron traveling across the
distance between electrodes is given by
The photocurrent is written as
where v = velocity of charge, Q = total photoelectron charge,
and
25
The equivalent circuit for the biplanar diode is shown in
figure 26. The output voltage is
V = 2^O ^2 V
= 2^
-t
t - T(l-e , 0 < t < T
-T (t-T)
T(l-e , T < t
where x = RC^. A graph of is shown in figure 2 7. By the
use of the planar vacuum photodiode, it is possible to generate
electrical pulses having large peak voltages with risetime of
100 ps.
7.4 Optical Rectification
The nonlinear optical effect induced by a strong optical
beam in a crystal has a component of DC electrical polariza-
tion; the production of the electrical polarization is called
optical rectification [25, 26]. The DC polarization induced by
light is given by
Pi(0,lY) = X2i.^C0,a)-to) EjCa) ,lY)E*C^,lT) ,
that is, proportional to optical power. The induced current
is represented by
. _ dP _ Y dl^or " 3t " -^2
where P is the integrated polarization over the volume of a
crystal and I is optical power. Typical materials available
26
for the optical rectification are KDP, ADP , LINbO^ and LiNbO^,
etc. each of which lack the inversion symmetry in the crystal
structure. The broadband response of the optical rectification
effects coupled with an appropriate transmission line and in-
tense picosecond laser pulses suggests the use of the effect
for generating picosecond electrical pulses.
27
ACKNOWLEDGMENT
by T. Honda
I would like to extend thanks to Dr. R. Sangster and
Dr. H. Altschuler for the administrative actions which enabled
this work. I have been greatly impressed and stimulated by
the research activities in the NBS Pulse and Time Domain Sec-
tion, 2 72.20. Also, I would like to thank Dr. Nahman for many
stimulating discussions and lectures and for his interest and
encouragement during this work. It is also a pleasure to
acknowledge the support of Dr. W. McCaa and the technical
assistance of Mr. T. Whittemore in the electronics work.
I would like to acknowledge and thank Dr. R. Lawton for
the helpful discussions on pyroelectric devices. I am indebted
to Mr. K. Wilson who has constructed many devices for this
work.
Finally, I am indebted to Dr. D. Jennings, NBS 271.00 ,
who lent me various optical devices. I would like to grate-
fully thank Dr. R. Smith, NBS 2 71.00, for the many helpful
discussions and suggestions, and furthermore, for the col-
laboration in the experiment on the vacuum photodiode responses.
28
REFERENCES
[I] A.J. DeMaria, W.H. Glenn, Jr., M.J. Brienza, and M.E.Mack, "Picosecond Laser Pulses," Proc. IEEE, Vol. 57,No. 1. pp. 2-25, January 1969.
[2] F.T. McClung and R.W. Hellwarth, "Gaint Optical Pulsesfrom Ruby," J. Appl. Phys
. , Vol. 33, pp. 82 8-829, March1962.
[3] R.W. Smith, "Mode-Locking of Lasers," Proc. IEEE, Vol. 58,No. 9, pp. 1342-1357, September 1970.
[4] C.C. Cutler, "The Regenerative Pulse Generator," Proc.IRE, pp. 140-148, February 1955.
[5] S.E. Schwarz, "Theory of an Optical Pulse Generator,"IEEE J. Quantum Electronics, Vol. QE-4, No. 9, pp. 503-514, September 1968.
[6] J.R. Creighton and J.J. Jackson, "Simplified Theory ofPicosecond Pulses in Lasers," J. Appl. Phys., Vol. 42,No. 9, pp. 3409-3414, August 1971.
[7] N.G. Bosov, P.G. Kriukov, V.S. Letokhov, and Y.V. Senatskii,"H-12-Generation and Amplification of Ultrashort OpticalPulses," IEEE J. Quantum Electronics, Vol. QE-4, No. 10,pp. 606-609, October 1968.
[8] D.J. Kuizenga and A.E. Siegman, "FM and AM Mode-Lockingof the Homogeneous Laser -- Part I: Theory," pp. 694-708, "Part II: Experimental Results in a Nd: YAG Laserwith Internal FM Modulation," pp. 709-715, IEEE J. QuantumElectronics, Vol. QE-6, No. 11, November 1970.
[9] K. Patek, "Glass Laser," London Iliffe Books.
[10] M.A. Dugvay, et al. , "Study of the Nd: Glass Laser Radia-tion," IEEE J. Quantum Electronics, Vol. QE-6, pp. 725-743, November 19 70 .
[II] D. Von Der Linde,"Experimental Study of Single Picosecond
Light Pulses," IEEE J. Quantum Electronics, Vol. QE-8,No. 3, pp. 328-338, March 1972.
[12] E.M. Garmier and A. Yariv, "Laser Mode-Locking SaturableAbsorbers," IEEE J. Quantum Electronics, Vol. QE-3, No.6, pp. 222-226, June 1967.
29
W.R. Hook, et al., "Xenon Flashlamp Triggering for LaserApplication," IEEE Transaction on Electron Devices,Vol. ED-19, No. 3, pp. 308-314, March 1972.
M. Hercher, "An Analysis of Saturable Absorbers," AppliedOptics, Vol. 6, No. 5, pp. 347-954, May 1967.
A. Penzkofer, et al., "The Intensity of Short LightPulses Determined with Saturable Absorbers," OpticsCommunication, Vol. 4, No. 5, pp. 377-379, January 19 72.
J. A. Giordmaine, et al., "Two-Photon excitation offluorescence by picosecond light pulses," Appl. Phy.Lett., Vol. 11, pp. 216-218, October 1967.
W.D. McCaa, Jr., and N.S. Nahman, "Generation of ReferenceWaveforms by Uniform Lossy Transmission," IEEE Trans-action on Instrumentation and Measurement, pp. 382-390,Vol. IM-19, No. 4, November 19 70.
E. Fatuzzo and W.J. Merz , Ferroelectricity , John Wiley§ Sons, 1967, pp. 63-76.
J.R. Alday, "Millimeter Wave Detectors Using the Pyroelec-tric Effect," IEEE Trans, on Electron Devices, Vol. ED-16,No. 6, pp. 598-601, June 1969.
R.A. Cowley, et al., "Dielectric Response in PiezoelectricCrystals," J. Physics-C, Vol. 4, No. 10, pp. L-203,July 19 71.
M. Simhony and A. Shavlor, "Pyroelectric Voltage Responseto Step Signals of Infrared Radiation in TriglycineSulphate and Strontium Barium Niobate," J. Appl. Phys .
,
Vol. 42, No. 10, pp. 3741-3744, September 1971.
L.K. Angers on and B.J. McMurtry, "High Speed Photo-detectors," Proc. IEEE, Vol. 54, No. 10, pp. 1335-1349,October 1966.
H. Melchior, M.B. Fisher, and F.R. Arams, "Photode tectorsfor Optical Communications Systems," Proc. IEEE, Vol. 58,No. 10, pp. 1466-1486, October 1970.
P. J.R. Laybourn, "Photodetectors for Laser Application,"Optics and Laser Technology, pp. 76-82, 1971.
30
[25] M. Bass, P. A. Franken , J.F. Ward and G. Weinreich, "Opti-cal Rectification," Phys . Rev. Letters, Vol. 9, No. 11,
pp. 446-448, December 1962.
[26] M.J. Brienza, A.J. DeMaria, and W.H. Glenn, "OpticalRectification of Mode-Locked Laser Pulses," PhysicsLetters, Vol. 26A, No. 8, pp. 390-391, March 1968.
[27] R.L. Smith and T. Honda, "The Optical Impulse Response ofBiplanar Vacuum Photodiodes , " to be published.
31
Table 1 PROPERTIES OF MODE -LOCKING DYES
DYE
ABSORPTION PEAK
SATURATION POWERDENSITY
RELAXATION TIME*
DAMAGE THRESHOLD(Relative Unit)
9740
1.045 vim
40 MW/cm^
25 'V. 35 X lO"-'-^ Sec
15
9860
1.051 ym
50 MW/cm^
6 9 X lO"-^^ Sec
43
32
Piww
o
C/2
m
ou
< SC3
CO
mH
o
<;
<I—
(
OhO
I—
I
o
PQ 2I-H O> S
2o
woPi<
hH
OOCJ
<E-"
I—
I
CJ
HCOiz:
ou
I
owH
uPi1—1
uCJ
CJ 2HH PiH WCO uD CJ•-3 h-l
Q Pi< H
HCO>HCO
wQCJ
HHH<
UPi(—
I
CJ
C3
I—
I
O
CO<C5
£3
o
PQ<HCO
PiH WCJ CO
5 <Pi
CO Ow wQ
CJPC ol-< I
oCO SwCO
<CJ
H2
2CJI—
<
CO
PiO<oCO
PimH
rsi
2OI-H
E-
OHCOHHQ
§PiwCO< <
CJI—
I
Ho
0
oPi tJU
oPiwCO
•J
o
2OI—
I
<HUPPL,
<cypio
Ph-o
CO
Piooa,
CJ uI—
I
PiE-CJm>-i
m
CJ
PiwCJCJI—
I
PiH
I—
t
CJPi m< ^uCO
PQ
OPiH
<CJI—
I
m>-Q
o
OH
CJmQ
COPim
O
COmuI-H
>mQ
oCOmCJ<PL,
Pi
CO
2OHCO
Q
33
EXPANDOR
DELAY
FILTER AMPLIFIER
OUTPUT
REFLECTOR
(a) System Model.
DELAY =2 L
(b) Laser Pulse Generator.
REFLECTOR
OUTPUT
Figure 1 A MODEL FOR ELECTRONIC AND OPTICAL
REGENERATIVE PULSE GENERATORS.
34
a =
a
1 +
s INTENSITY I
Figure 2a OPTICAL NONLINEAR ELEMENT FOR
MODE LOCKED LASER.
NUMBER OF PASSES
01. 4
= 0.7
Pulse
Figure 2b PULSE SHARPENING DUE TO REPEATEDTRAVERSALS OF THE FEEDBACK LOOP.
35
CO.
r(t) or F(co)
©Liinear Operator
Laser Linewidth
Linear Gain- Losses
CA
Propagation
Delay
NonlinearElement
Power Law
Figure 3 LASER MODEL FOR CUTLER ANALYSIS.
36
o
uo
ovOCO
o
in
oCO
Q U
ft;
HO
S: P i w ^^ ^ 5 Q 3S o w
o
o
w
<;
CO
QW14
uo
wQO
CD
•H
37
200 M s/ div
Conventional
50 ns/div
dye mode-locked
Figure 5 CONVENTIONAL LASER AND MODELOCKED LASER OUTPUTS.
38
COU
oa:
Hu
w
:[:
CO
S
OJ
WCO
i
0)
39
Conventional Mode-locked
Figure 7 LASER BURN PATTERNS.
40
Shot #1
Shot #3
Figure 8 SHOT TO SHOT INSTABILITY OF MODELOCKING .
9860 dye(T = 80%), 100% & 70% mirrors,
800 J constant, time scale = 10 ns/div.
41
800 J
r
-"1-4
760 J
610 J
Figure 9 EXCITATION ENERGY DEPENDENCE OF
MODE LOCKING.
9860 dye (T = 80%), 100% & 70% mirrors.
Shot #15
Shot #29
Shot #40
Figure 10 FATIGUE OF DYE.
9860 dye (T = 80%), constant 800 J input.
43
44
Strong Focusing '.
(high power density)
less absorption
Weak focusing:
(low power density)
more absorption
Figure 12 OPTICAL ATTENUATION VERSUS INTENSITY
OF 9860 DYE (T = 6%).
45
(•/. ) NOISSIWSNVdi
46
s<
Q _i
lUI—
<
»—oI
o
J
I
top?ta
\o
°\
\
t—oX o5
I/)
\ o
A-
V
\ °
3
I—
z8
ilU>-o
ooo o00
o o oin
(•/.) NOISSIWSNVai
47
(0
0)
in
CO
(M
OO o o o oin
00 O
OzoI—
I
to
zo
u
OX)•i-i
3cq
WQZ
CO CO
<
CO
>W
Q
(%) NOISSIP^SMVHX
48
49
(a) A train of pulses missing a single pulse. 20 ns/div.
(b) Single pulse extracted from a train of pulses. 5 ns/div.
Figure 17 SINGLE PULSE GENERATION WAVEFORM
50
51
^I
185 ps ^TPF Pattern
Pulse Wavetrain
20 ns / div.
Figure 19 TWO PHOTON FLUORESENCE RESULTS.
52
2 ps
Figure 20 TWO PHOTON FLUORESENCE RESULTS.
53
Unit rampgenerator
y(s) z^(8)
,Tfanstnisslon bandwidth- limited I
Iby planar skin effect and Debye
j
1 dielectric losses, i
(a) Present NBS reference waveform generator.
Mode Locked
Nd"*"^ Glass
Laser
Measurementon PulseDuration
ModeledDetector
Impulse response
(Transfer function)
to be predictable
(b) Proposed future NBS reference waveform generator.
Figure 21 GENERATION OF PICOSECOND PULSED
WAVEFORM OF KNOWN SHAPE.
54
Figure 22 EQUIVALENT CIRCUIT FOR A PYROELECTRICDETECTOR
.
55
I
step
impulse
^
Figure 23 PYROELECTRIC DETECTOR WAVEFORMS.
56
Figure 24 PYROELECTRIC DETECTOR.
57
U d
Q
anode photocathode
Figure 25 BIPLANAR PHOTODIODE.
Figure 26 EQUIVALENT CIRCUIT FOR BIPLANAR
PHOTODIODE.
Figure 2 7 IMPULSE RESPONSE OF BIPLANAR PHOTODIODE.
58
.FORM NBS-IKA (1.71)
U.S. DEPT. OF COMM. 1 . PUBLIC ATION OR RE PORT NO. 2. Gov't AccctslooBIBLIOGRAPHIC DATA NBSIR 74-360
SH E E T
3. Recipient's Accession No.
4. TITLE AND SUBTITLE^ 3
A Dye Mode-Locked Nd Glass Laser for GeneratingElectrical Reference Waveforms
5. Publication Date
September 1972
7. AUTHOR(S)
T. Honda and N.S. Nahman1. |*et£ot(mag OtganintioB
9. PERFORMING ORGANIZATICffJ NAME AND ADDRESS
NATIONAL BUREAU OF STANDARDS, BoulderLabs.DEPARTMENT OF COMMERCEBoulder, Colorado 80302
l6. Project/Taslc/Work Unit No.
:722159 & Z/Zo±by11. Contract/Grant No.
12, Sponsoring Otganization Name and Address
National Bureau of StandardsDepartment of CommerceBoulder, Colorado 80302
13. Type of Report & PeriodCovered
NBSIR FY 7 2
14. Sponsonag Agency Code
15. SUPPLEMENTARY NOTES
16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significantbibliography or literature survey, mention it here.)
The theory, design, construction, operation and stability of dye
saturable dye, single pulse generation and two-photon fluorescence areexperimentally studied. For the application of picosecond optical pulsesto the field of the baseband pulse measurements, three types of modelabledetectors are described which have possible applications for thegeneration of electrical reference waveforms.
17. KEY WORDS (Alphabetical order, separated by semicolons)
Glass laser; laser; mode-lock; Nd"*"^; picosecond; reference waveform
18. AVAILABILITY STATEMENT
r 1 UNLIMITED.
IXl FOR OFFIQAL MSTRIBUTION. DO NOT RELEASETO NTIS.
19. SECURITY CLASS(THIS REPORT)
UNCLASSIFIED
21. NO. OF PAGES
20. SECURITY CLASS(TfflS PAGE)
UNCLASSIFIED
22. Price
USCOMM-OC ••244.P7Ilacoioi »L
